This invention relates to communicating channel estimates, and specifically to determining when to transmit a channel estimate.
Multiple element antenna arrangements have been the subject of intense research over the past few years as a way of increasing the coverage or capacity of a wireless link. (See “Introduction to Space-Time Wireless Communications” by Paulraj, Nabar and Gore). The generic case is where both the transmitter and receiver(s) on a link have multiple antenna elements all operating on the same radio channel and advantage is taken of the spatial and time differences between them. This system is known as MIMO (multiple input, multiple output), and is typically based on spatial multiplexing.
There are several ways in which MIMO can be implemented, but two general categories can be distinguished.
In a first type of system, the transmitter uses no knowledge of the channel in determining what signals to transmit via which antennas. This type of system is called open-loop MIMO. In open-loop MIMO the transmitter typically transmits equal energy from each antenna element and applies symbol coding that is resilient to any channel conditions. An example of such coding is the Alamouti coding specified by the WiMAX forum.
In a second type of system the transmitter uses knowledge of the channel to determine what signals to transmit via which antennas. That knowledge is gained through feedback from the receiver. This type of system is called closed-loop MIMO. In a closed-loop MIMO system, the transmitter can use algorithms to set energy allocation and coding across the antennas in dependence on its knowledge of the channel. This can increase capacity and/or signal to interference ratio. However, in order to achieve these gains it is necessary for the receiver to feed back information about the channel. That feedback uses some capacity, and so there is a need to balance the gains obtained from more detailed channel knowledge against the capacity overhead that is lost to sending channel feedback information.
To address this balance, a considerable amount of research has looked for ways of maximising the coverage and capacity of closed-loop MIMO systems using a minimum of feedback. The problem with this approach is that the characteristics of the channel change when any part of the channel alters: for example when the transmitter or the receiver moves, or when an object moves in the propagation path. Therefore, when a minimum of feedback is employed the coverage and capacity are not increased greatly beyond open-loop conditions.
The 3GPP (Third Generation Partnership Project) have adopted a code-book method in their LTE (long term evolution) system. In the code-book system a label is sent to the transmitter from which the transmitter can look up the multi-path matrix that most closely resembles the channel, similar to items in a catalogue. However, this system is restrictive to a certain number of channel types and is far from optimum. (See “Grassmannian Beamforming for MIMO Wireless Systems” by David Love for a general discussion of code-book methods). Another approach to reducing the bandwidth needed to provide channel feedback information is to assess the channel periodically at the receiver and to send back to the transmitter a channel condition number (see for example “Switching Between Diversity and Multiplexing in MIMO systems” by Heath and Paulraj, 2005).
There is a need to improve the balance between gains that can be had from making use of channel information at a spatial multiplexing, MIMO, transmitter and the loss of bandwidth that results from feeding back channel information to the transmitter.
According to one aspect of the present invention there is provided a transceiver for operation in a spatial multiplexing antenna communication system, the receiver comprising signal processing equipment configured to: form a channel estimate for a channel between the transceiver and a second transceiver; estimate capacity of the channel in dependence on the channel estimate; determine in dependence on the estimated channel capacity whether to transmit data indicating that estimate to the second transceiver; and cause the transceiver to transmit data indicating that estimate to the second transceiver only if the said determination is positive.
According to a second aspect of the present invention there is provided a method for controlling a transceiver for operation in a spatial multiplexing antenna communication system, the method comprising: forming a channel estimate for a channel between the transceiver and a second transceiver; estimating capacity of the channel in dependence on the channel estimate; determining in dependence on the estimated channel capacity whether to transmit data indicating that estimate to the second transceiver; and transmitting data indicating that estimate to the second transceiver only if the said determination is positive.
The said determination may comprise: estimating the capacity of the channel if the second transceiver were to know the channel estimate (“a”); and estimating the capacity of the channel if the second transceiver were to have no knowledge of the channel (“b”); determining that data indicating the channel estimate should be transmitted if (a−b) exceeds a predetermined threshold. The predetermined threshold may be zero, or may be greater or less than zero. The transceiver may be configured to estimate the value of a by means of a water-filling algorithm.
The transceiver may be a MIMO transceiver and/or an ODFM transceiver.
The transceiver may be configured to determine when to form the channel estimate in dependence on the frequency with which channel estimates have previously been transmitted to the second transceiver.
The said data indicating the channel estimate may be data expressing the change in the channel estimate since it was last transmitted to the second transceiver.